Detection/Diagnosis https://doi.org/10.29289/259453942020V30S1010 MONOPHASIC BREAST SYNOVIAL SARCOMA – CASE REPORT Juliana Lopes de Aguiar Araujo1, Ubiratan Wagner de Sousa1, Fernanda Mabel Batista de Aquino1, Luisa Gurgel de Lira2, Iris Maria Rodrigues de Andrade Almeida3 1Liga Norte Rio-Grandense Contra O Câncer – Natal (RN), Brazil. 2Universidade Potiguar – Natal (RN), Brazil. 3Universidade Federal do Rio Grande do Norte – Natal (RN), Brazil. Introduction: Breast synovial sarcoma is extremely rare, with few cases described. It corresponds to 6–9% of soft tissue sarcomas and is more frequent in extremities (80%), trunk (8%), and abdomen (7%) in young adults. It usually does not affect the breast. Objectives: To report a rare case of monophasic breast synovial sarcoma and provide data for the glo- bal literature. Method/Case report: G.S.B., 97 years old, presented a 7 cm nodule in the left breast and negative axillary nodes. Ultrasound (US) revealed a heterogeneous nodule of 6.0 x 5.5 cm, BI-RADS 5. She did not have mammography. Core biopsy showed spindle cell neoplasm. Immunohistochemistry (IHC) indicated mesenchymal lesion, without differentia- ting stromal components of fibroepithelial tumor from the mesenchymal lesion. Rapid growth with ulceration and tumor bleeding were identified. Urgent mastectomy showed a malignant neoplasm of spindle cell pattern and high grade, with 12 cm, involved lateral margin, and 19 negative axillary lymph nodes. IHC of the surgical specimen indicated monopha- sic synovial sarcoma. Before the wide excision, she had a rapidly progressive recurrence in the sternum, making it non- -resectable. During radiotherapy (RT), local progression was identified. She has been receiving chemotherapy (CT) with ifosfamide and adriamycin. No evidence of distant disease was found after 9 months of diagnosis. Results/Discussion: Synovial sarcoma corresponds to approximately 0.06% of all breast neoplasms, originating from their mesenchymal tis- sue, with variable epithelial differentiation. The term synovial sarcoma is inadequate, deriving from its frequent juxta-arti- cular location. Its incidence is approximately 1.5 per 1 million individuals, with a mean age of 32 years and a male:female ratio of 1.2:1. The main histological subtypes are: classic biphasic and monophasic. Translocation t(X;18) (p11.2; q11.2) and expression of SYT/SSX gene fusion are present in more than 95% of cases. IHC shows an intense expression of vimentin and CD99, and focal of Bcl2, EMA, CKAE1-AE3, actin, and desmin, as well as negativity for S100, cytokeratins, hormone receptors, myosin, and caldesmon. The differential diagnosis is made with other spindle cell entities, such as fibromato- sis, solitary fibrous tumor, myofibroblastoma, metaplastic carcinoma, and other sarcomas. Synovial sarcoma has a mode- rate response to chemotherapy with anthracyclines. The treatment includes wide surgical resection and RT. Metastases occur in about 50% of cases and are present at diagnosis in 16% to 25%; they are more frequent in the lung (75%), regional lymph nodes (15%), and bones (10%), tending to late recurrence and metastases. The 5-year disease-free survival is 60%. Conclusion: The heterogeneity of the disease and its low incidence hinder prospective studies addressing therapeutic options with better long-term results. 10 Mastology, 2020;30(Suppl 1):10.